1. Field of the Invention
This invention relates to multi-component system lithium phosphate compound particles having an olivine structure, the manufacturing method thereof and a lithium secondary battery employing the lithium phosphate compound particles as a positive electrode material.
2. Description of the Related Art
A lithium secondary battery employing, as a negative electrode active material, metallic lithium, lithium alloys or a material which is capable of absorbing and desorbing lithium ion is characterized by a high voltage and by excellent reversibility. Especially, in the case of a lithium ion secondary battery wherein a composite oxide comprising lithium and a transitional metal is employed as a positive electrode active material and a carbonaceous material is employed as a negative electrode active material, since the battery is lighter in weight and larger in discharging capacity as compared with the conventional lead secondary battery or with the conventional nickel-cadmium secondary battery, it is widely employed as a power source for various electronic devices.
As for the positive electrode active material for a lithium ion secondary battery which is generally employed at present, a compound such as LiCoO2, LiNiO2, LiMnO2 or LiMn2O4 is mainly employed. However, the reserve of cobalt and nickel is small and, moreover, the output of these metals is limited to a limited area. Because of these reasons, when it is desired to employ these materials as a positive electrode active material for a lithium ion secondary battery whose demand is expected to be further increased from now on, the employment of materials containing these metals are restricted not only in the respect of price but also in the respect of stabilized supply of raw materials. Further, in view of safety also, since these active materials are high in reactivity, the employment of these active materials may raise problems. Further, although manganese is a relatively cheap material, the employment of a material containing manganese as a positive electrode active material may give rise to problem of stability in cycle characteristics of battery.
For these reasons, there has been proposed in JP-A 9-134724, JP-A 9-134725 and JP-A 2001-085010 the employment, as a positive electrode active material of lithium secondary battery, of lithium iron phosphate or partially substituted lithium iron phosphate containing other element(s) substituting for a portion of iron. In these materials, iron which is large in output, cheap in price and promising in stable supply is employed as a raw material.
However, since these lithium phosphate compounds having an olivine structure are very high in electric resistance as compared with lithium metal oxide such as LiCoO2, etc., which has been conventionally employed, the resistance polarization is caused to increase during the charging/discharging operation, thereby raising problems that it is impossible to obtain a sufficient discharge capacity and receivability for charging the battery. These problems become more prominent on the occasion of carrying out the charge/discharge of large electric current.
As one of the methods for solving these problems, there has been studied to finely pulverize the particle of lithium phosphate-based material having an olivine structure so as to increase the reactive surface area of the particle and to facilitate the diffusion of lithium ions, thereby shortening the distance through which electrons are enabled to flow in the interior of the particles of lithium iron phosphate-based material. However, the finely pulverized particle of lithium phosphate-based material having an olivine structure is characterized in that it tends to easily generate secondary aggregation on the occasion of mixing it with a conductive material such as carbon black, etc., during the process of manufacturing electrodes. If this secondary aggregation is caused to occur, since the particles of lithium iron phosphate-based material would be point-contacted with each other or with the electrically conductive material in the aggregated secondary particle, there is a problem in that it is impossible to obtain sufficient collecting effects, thereby greatly increasing electric resistance. For this reason, even if the charge/discharge of battery is performed, the active materials existing at a central portion of the aggregated particle are incapable of executing conduction of electrons, thus causing the deterioration of the charge/discharge capacity of the battery.
Furthermore, since the finely pulverized particle has a large surface area, the quantity of dissolution thereof into an electrolyte is liable to be increased, thus giving rise to a problem of long term stability. Further, due to the enlarged surface area, the quantity of a dispersing medium which is required for the preparation of a slurry on the occasion of preparing electrodes is caused to increase, thereby raising various problems such as the difficulty to secure a sufficient quantity of coating of lithium phosphate-based material, easy generation of cracking during the drying process thereof, and the difficulty of increasing the capacity of battery due to the difficulty in achieving a sufficient degree of compression.
In view of overcoming these problems, there has been proposed an idea of applying fine particle of a material which is electrically conductive and more noble in redox potential than lithium iron phosphate-based material such as silver, carbon, platinum, palladium, etc. (see for example JP-A 2001-110414).
There has been also proposed a method wherein carbon is used as an electrically conductive agent, and a solution, a fluid dispersion or a suspension containing a Li source, a Fe source, a P source and a C source is sprayed into an atmosphere of high temperatures to obtain a precursor, which is then heat-treated in a reducing atmosphere or in an inert atmosphere, thereby enabling carbon to uniformly disperse on the surface of the particles of lithium iron phosphate-based material (see for example JP-A 2005-116392).
There has been also known, as a method of further enhancing the electric conductivity among the particles, to employ lithium iron phosphate-based composite oxide/carbon composite wherein the surface of the particles of LiFePO4 is covered with a carbonaceous material and an average particle diameter of the carbon composite is confined to 0.5 μm or less (see for example JP-A 2003-292309).
Meanwhile, the redox potential of iron existing in lithium iron phosphate is lower than other elements. For example, as compared with ordinary lithium cobaltate, the redox potential of iron is known as being lower by 0.2 V. Therefore, in an attempt to minimize electric resistance and to increase electric potential, a method has been proposed wherein one or more compounds containing a metal selected from the group consisting of iron, cobalt, manganese, nickel, copper and vanadium is allowed to react with one or more compounds containing lithium, the resultant reacted body being subsequently sintered at a prescribed temperature (see for example JP-A 2003-157845).
Further, there has been proposed a method wherein a portion of iron of lithium iron phosphate is replaced by cobalt (see for example Journal of Power Sources 146 [2005], pp. 580-583).